Construction of a Soul

Every person is made up of three distinct souls. The souls intertwine to form a complete person, and all three souls must be strong and present for a person to enjoy good health.

Transference and Transformation

Though only humans have a tausai, and all living things have a lua and a panai. Things like plants and molluscs have very different luas, because their intake of oxygen is not as obvious as breathing with lungs, but they still have it. The panai is very small and faint in some things (such as plants) so that they are difficult to notice, but they are still there.
Luas and panai are closely related, but they are separate and, with magic, it is possible to move the panai to a different body and a different lua. Experiments with this have revealed most of what is known about how animal souls work:

• If a human panai is moved into the body of an animal (with the animal lua), the human mind will quickly degenerate to base animal instincts. They will die within the expected lifespan of the animal.
• If an animal's panai is moved into a human body (with a human lua), the animal's mind will expand and became capable of human-level intelligence. It will retain instincts from its life as an animal, but after a short period of adjustment and learning, it will be fully capable of speech and rational thought

These cases underline the basic rules: a lua determines what a being can be, while the panai determines who they are.

Transformation

Both of those are a form of transference - moving a panai from one body to another. It is impossible to transfer a lua - they are bound to the body they inhabit until death. Removing a lua causes immediate death. The only way to get a lua separate from its body is to catch one as it leaves the body upon death.
However, it is possible to transform the physical body of a person. This is a powerful branch of magic, and doing it to a person other than yourself is even more challenging (and also considered a grave crime if it was non-consensual.) At least 95% of transformative magic involves a person changing their body to became the animal represented by their tausai. The nature of the tausai gives a huge helping hand in making the magic work.
When a person's body changes, the lua that is tied to it remains the same. For this reason, the person can be in the body of an animal but retain their human mind, and they will age and die on a human time frame. Likewise, an animal transformed into a human will have the same level of intelligence they did as an animal, and will only live as long as their animal lua determines.